Stereotactic surgery is the gaining of precise access to a specific point in the body, especially the cranium, of a patient, by reference to an external three dimensional coordinate system. With the development of magnetic resonance imaging ("MRI") and the precise imagining it provides, stereotactic surgery is becoming a diagnostic and therapeutic procedure of choice for many disorders involving the intracranial cavity. By way of illustration, an MRI study can be used to form a three dimensional picture of the brain which defines the positional relationship of neurological structures or which accurately localizes a lesion in the brain. With the aid of stereotactic apparatus the precise position of the lesion or other site within the cranium can be specified with reference to an external reference source that is fixed with respect to the skull. Once the location of the target has been determined with respect to the reference source, inspection and treatment can be carried out more precisely because location of the target within the cranium is accurately specified and precisely accessible.
By way of more specific illustration, Carol U.S. Pat. No. 4,805,615, issued Feb. 21, 1989, discloses a method and apparatus for performing stereotactic surgery, in which there is established a first predetermined geometric relationship between a positioning fixture mounted to the patient's skull, and a scanning table surface upon which the patient is positioned. That geometric relationship is then duplicated in an external or "phantom" fixture. The phantom fixture duplicates the same geometric relationship, but outside the cranium. The phantom fixture includes means for receiving at least a portion of the positioning fixture, and means for establishing a second predetermined geometric relationship between that portion of the positioning fixture and the phantom fixture, so that the second geometrical relationship is identical to the first. The position and slope of the skull at the location where the positioning fixture is attached to it can be duplicated within the phantom fixture. With the target thus pinpointed, it can thereafter be accessed more exactly.
A pending application, Carol Serial No. 111,987, filed Oct. 22, 1987, now U.S. Pat. No. 4,955,891, entitled "Method and Apparatus for Performing Stereotactic Surgery," discloses a related device including an arcuate fixture for use with the phantom fixture of the patent, whereby a medical instrument on the arc fixture can be precisely aligned with the target from a plurality of locations on the skull. The disclosures of the Carol patent and application are incorporated herein by reference.
In various neurosurgical procedures it is desirable to precisely insert or implant a medical instrument or device (referred to as an "intracranial device") at a specified location in the cranium. For example, an electronic device such as a so-called "depth electrode" may be inserted to measure electrical activity at a given point in the brain, or reinserted to the same point at a subsequent time; or a radioactive seed may be implanted at a precise location for treatment of a lesion in the brain.
The stereotactic devices of the Carol patent and application make it possible to establish an imaginary radial axis which extends a predetermined distance directly to the target area in the skull, from an external point. Given such precise location specifying capability, there is also a need for means to insert an intracranial device precisely along a preestablished radial axis from outside the skull, and to implant it at the specified target depth. In cases where the device is removed after treatment, it is desirable to be able later to reinsert the device along the same axis, to the same point. Such insertion involves forming an opening in the cranium; then inserting the device, which may be delicate and/or flexible, to the desired location along a predetermined direction and to a predetermined depth, through the opening. Moreover, it is preferable that any electrical wires connected to the device remain connected throughout the implanting procedure. Thus if a cannula or sleeve is used to guide the insertion of the device, the cannula should be removable from the device without requiring disconnection of any wire to it.